1. Field of Invention
The present invention relates in general to a composition of matter in which resins are polymerizable using non-volatile initiating species. More particularly, the present invention relates to compositions that are especially adapted for resin formulations (e.g., polyphenylene ether (PPE) and modified PPE resin formulations) used in printed circuit board laminates. These compositions are made into prepreg dielectric sheets or coated on a substrate and cured to form dielectric laminate structures for circuit packaging structures.
2. Background Art
One conventional technique for forming laminate circuit structures is to provide a sheet of what is referred to as prepreg, which is a glass cloth impregnated with a resin solution which subsequently is either dried or at least partially cured.
Also, a similar type of resin is sometimes used without glass cloth and coated onto metal, such as copper, for use as build up layers. These are commonly referred to as resin coated copper (RCC) or polymer coated copper (PCC). Such prepregs are then laminated with copper foils forming cores. Cores are then processed further, such as forming vias and circuitry thereon, and then laminated with additional prepregs and copper foils to form composite laminate structures.
The resin used to prepare these printed circuit board laminates typically includes a polymer, flame retardant, viscosity modifiers and an initiator. All of the components in the resin are non-volatile with the exception of the free radical initiator. Volatile loss of the initiator can occur during several of the steps in the preparation of the laminates (e.g., during resin preparation, during impregnation of the glass cloth, during coating of the copper, and/or during shipment, handling and curing of the prepreg). This volatile loss of the free radical initiator can cause significant changes to the physical properties of the laminate. Specifically, volatile loss of the initiator can change the degree of cross-linking in the laminate, thus changing the resultant mechanical properties. This can also lead to reproducibility problems as the laminate can outgas more or less of the initiator, depending on the environment to which it is subjected.
Particularly useful resins for forming prepregs are described in U.S. Pat. No. 6,352,782 B2, often referred to as PPE, assigned to General Electric Company, hereinafter sometimes referred to as the GE patent, and in U.S. Pat. No. 5,352,745, often referred to as modified PPE or APPE, assigned to Asahi Kasei Kogyo Kabushiki Kaisha of Tokyo, Japan, hereinafter sometimes referred to as the Asahi patent. Both of these patents are incorporated herein by reference in their entirety.
The PPE resin as described in the GE patent is a reactively end capped poly(phenylene ether) compound cured with certain unsaturated compounds for synthesizing resins ideally adapted for impregnating fibrous reinforcement in the manufacture of circuit boards.
The modified PPE as described in the Asahi patent is a curable polyphenylene ether resin composition comprising a reaction product obtained by reacting a polyphenylene ether with an unsaturated carboxylic acid or an acid anhydride and at least one cyanurate. Generally speaking, these compositions include about 98% to about 40% by weight of a curable polyphenylene ether resin comprising a reaction product obtained by reacting a polyphenylene ether with an unsaturated carboxylic acid or an acid anhydride, and 2% to 60% by weight, based on the total weight of this and the previous components, of at least one cyanurate selected from the group consisting of triallyl isocyanurate (TAIC) and triallyl cyanurate. These compositions include an initiator. Generally speaking, the initiator is a peroxide of a low molecular weight compound, i.e., below about 700 grams per mole. These peroxides are described in the Asahi patent at column 13, lines 10-23, and in the GE patent at column 12, lines 19-28.
An earlier PPE composition is described in U.S. Pat. No. 5,218,030, assigned to Asahi Kasei Kogyo Kabushiki Kaisha of Tokyo, Japan. It describes the use of poly(phenylene ether) containing pendant allyl or propargyl groups, triallylcyanurate or triallylisocyanurate, and optionally an antimony-containing flame retardant. Other formulations replace this antimony flame retardant with bromine containing compounds.
Several problems have been encountered when using the PPE or modified PPE compositions incorporating low molecular weight initiators. (Hereinafter, the term PPE may be used for both PPE and modified PPE.) First, one problem is that the prepreg has component volatility characterized by the volatilization of the low molecular weight peroxide initiators. Second, following lamination, there is a marked out-gassing which has been attributed to the breakdown components of the low molecular weight initiator that do not enter into the reaction and are trapped in the matrix and outgas upon lamination. Third, the inability to laminate the material after the prepreg has been exposed to wet processing. It is believed that this is due to the fact that the low molecular weight initiator is driven off when the prepreg is heated to remove any absorbed water. The first and third of these conditions result in poor crosslinking and, thus, degraded material and final structure properties. The second of these conditions results in the propensity to delamination of the cured prepreg layers. The need to solve these problems to improve the commercial viability of the currently available PPE and APPE products was recognized in U.S. Pat. No. 6,734,259 B1, assigned to International Business Machines Corporation, hereinafter sometimes referred to as the IBM patent. The IBM patent, which is described below, is incorporated herein by reference in its entirety.
The prepreg resin described in the IBM patent includes a polymerization initiator comprised of a peroxide functionalized polymer that is fragmented by heat to a plurality of free radical moieties, and a relatively inert moiety having a molecular weight greater than about 1000. The high molecular weight polyperoxide material provided as an initiator is described in the IBM patent at column 4, line 27-column 5, line 34. According to the IBM patent, the polyperoxide is preferably a peroxidized polystyrene. The IBM patent depicts a general structure of a peroxide functionalized polymer that can be used. A polymer in the general structure depicted can be functionalized to incorporate peroxide groups in pendant side chains or at the end groups. In other respects, the prepreg resin described in the IBM patent is like those described in the GE patent and the Asahi patent. These compositions include about 98% to about 40% by weight of a curable polyphenylene ether resin, and 2% to 60% by weight, based on the total weight of this and the previous component, of triallyl isocyanurate. A bromated flame retardant is also typically included in these compositions.
Several problems have been encountered when using the PPE or modified PPE compositions incorporating the high molecular weight polyperoxide material described in the IBM patent as a polymerization initiator. First, one problem is that the physical properties of the prepreg (e.g., glass transition temperature Tg, mechanical properties, etc.) may change because the free-radical initiators are bound to the polymer as pendant functional groups (rather than being bound in the backbone of the polymer). Second, upon heating to cure the prepreg, the free radical initiators bound to the polymer prior to heating are degraded to generate only one initiating species (along with the relatively inert moiety). A plurality of free radical moieties are generated only if prior to heating the free-radical initiators are bound to the polymer as a plurality of pendant functional groups. Third, the generated initiating species is not bound to the polymer and, thus, is volatile. The generated initiating species (i.e., a number of t-butoxide moieties which will react with the TAIC to promote crosslinking) has a boiling point of 85° C., while the temperatures used to cure the prepreg typically exceed approximately 130° C. Because the generated initiating species is volatile, its effectiveness as an initiator is significantly reduced and the physical properties of the resulting material may change uncontrollably from batch to batch.
These problems are also encountered when the polymerization initiator described in the IBM patent is incorporated in other resin compositions (i.e., resin compositions other than PPE and modified PPE compositions). The PCB industry has recently migrated away from the traditional FR4 epoxy based resins (due to lead-free requirements and the higher soldering temperatures associated with tin-silver-copper alloys). Hence, current resin coatings are typically no longer comprised of FR4 epoxies, rather they are more likely to be bismaleimide triazine (BT) resins or polyphenylene oxide/triallyl-isocyanurate (PPO/TAIC) interpenetrating networks. Incorporating the high molecular weight polyperoxide material described in the IBM patent as a polymerization initiator into such other resin compositions introduces the same problems noted above.
Therefore, a need exists for these problems to be solved to improve the commercial viability of the currently available PPE and APPE products, as well as enhance the performance of other resin compositions that are used in printed circuit board laminates.